1. Field of the Invention
The present invention relates to organotitanium precursors for use as source materials for titanium dioxide in metal-organic chemical vapor deposition and methods for making such precursors. More particularly, the invention is an organotitanium precursor made of a xcex2-ketoester and a titanium glycolate, and dimer precursors derived from a reaction of the above organotitanium precursor and alcohol.
2. Description of the Related Art
Volatile organotitanium precursors are generally used as source materials for fabrication of thin films of ferroelectrics and paraelectrics such as PZT and BST by metal-organic chemical vapor deposition (MOCVD) as these precursors are constitutents of titanium dioxide thin film. Known titanium tetraalkoxide precursors are so sensitive to the atmosphere and moisture that they are easily decomposed or oligomerized. There is therefore a problem in the art as this sensitivity detracts from the very properties sought, namely, the volatile properties of the precursors.
Accordingly, in order to overcome this drawback, Ti(OPri)2(tmhd)2(wherein, OPri is isopropoxide, and tmhd is 2,2,6,6-tetramethyl heptanedionate) was developed in which two diketone compounds substitute for two alkoxides. However, because Ti(OPri)2(tmhd)2 is also decomposed or oligomerized by a disproportionation reaction during its use, the problem of volatility was not resolved. Additionally, the decomposition reaction of this precursor proceeds in a complicated two steps or more process wherein a thin film is obtained only at a high temperature above 460xc2x0 C. Further, humps and haziness appear on the surface of the deposited thin film, and the surface has protrusions which are not smooth, as shown in WO 00/37712, J. Electrochem. Soc., 146(10) 3783-3787 1999.
In 1998, the Japanese Asahi Denka company developed Ti(mpd) (tmhd)2(wherein, mpd is 2-methyl-2,4-pentane-diolate). Compared with Ti(OPri)2(tmhd)2, the decomposition reaction of Ti(mpd) (tmhd)2 is simpler and the drawback of thermal decomposition or oligomerization during its use, was resolved.
However, for depositing a titanium dioxide thin film using Ti(mpd) (tmhd)2, the temperature of the substrate has to be maintained at 480xc2x0 C. or higher to maintain a sufficient deposition rate allowing a deposition of thin film having a constant titanium composition ratio. If Ti(mpd) (tmhd)2 is used with traditional DRAM technologies allowing a temperature application of at most 470xc2x0 C., a Ti-deficient thin film can be deposited.
If Ti(mpd) (tmhd)2 is used to manufacture a thin film having a composition ratio of barium and strontium to titanium of 1:1 in the BST film, the deposition rate of this titanium precursor is low, and there is a need to maintain a composition ratio of barium to strontium to titanium precursor at 1:1:8 in a mixed solution of Ti(mpd)(tmhd)2, barium, and strontium precursors. Accordingly, there is a serious waste of titanium precursor. Further, Ti(mpd)(tmhd)2 precursor is a brown colored glass-like solid, and is sold in the form of solution dissolved in a solvent. Therefore, it is not easy to separate the pure precursor of solid phase and deal with it, as disclosed in Japanese Laid-Open Publication Nos. 11-255784 and 10-114781, and J. Mater. Res., 14(10), 3988-3994 1999.
It is an object of the present invention to provide organotitanium precursors which are highly adaptive and efficient as source materials for MOCVD, and manufacturing methods for the inventive organotitanium precursors.
Another object of the present invention is to provide organotitanium precursors having superior stability and volatility which can be used in uncomplicated decomposition reactions on substrates with a high deposition rate at low temperatures such as 470xc2x0 C. or lower, and providing an easy dealing, such that they are very useful as source materials for MOCVD.
The organotitanium precursor of the present invention is of Formula 1 as follows, 
where R1 and R2 can each be a straight- or branched chain alkyl group each having 1-8 carbon atoms, a cycloalkyl group, a phenyl group or a benzyl group, and R3 is a straight- or branched chain alkylene group having 2-13 carbon atoms.
The di-nuclear organotitanium precursor of the present invention has Formula 2 as follows, 
where R1 and R2 can each be a straight- or branched chain alkyl group each having 1-8 carbon atoms, a cycloalkyl group, a phenyl group or a benzyl group, R3 is a straight-or branched chain alkylene group having 2-13 carbon atoms, and R4 is a straight chain alkyl group having 1-8 carbon atoms.
The method for making the organotitanium precursor of Formula 1 comprises reacting a titanium tetraalkoxide, Ti(OR)4, wherein R is a straight- or branched chain alkyl group having 1-4 carbon atoms, with a diol of Formula 3. Next, a xcex2-ketoester of Formula 4 is added to the resulting reaction intermediate to form a reaction product. All unnecessary by-product ROH is removed. If a solvent was used in the initial step whereby reaction intermediate is formed from Ti(OR)4, such solvents are also removed with unnecessary by-product ROH. The reaction product is then distilled under reduced pressure. The structure of Formula 3 is HOxe2x80x94R3xe2x80x94OH, where R3 is a straight- or branched chain alkylene group having 2-13 carbon atoms, and Formula 4 has the following structure, 
where R1 and R2 can each be a straight- or branched chain alkyl group each having 1-8 carbon atoms, a cycloalkyl group, a phenyl group or a benzyl group.
The di-nuclear organotitanium precursor of Formula 2 is made with a process comprising the steps of reacting a Ti(OR)4, wherein R is a straight- or branched chain alkyl group having 1-4 carbon atoms, with a diol of Formula 3. To the resulting reaction intermediate is added a xcex2-ketoester of Formula 4 to form a mono-nuclear reaction product. All unnecessary by-product ROH is removed. If a solvent was used in the initial step whereby reaction intermediate is formed from Ti(OR)4, such solvents are also removed with unnecessary by-product ROH. The mono-nuclear reaction product is then distilled under reduced pressure. Either before or after the distilling step, an alcohol, R4OH, where R4 is a straight chain alkyl group having 1-8 carbon atoms, can be added to the mono-nuclear reaction product, thereby yielding di-nuclear reaction product.